1. Field of the Invention
The present invention relates to the recycling of vulcanized elastomers, such as vulcanized rubber found in tires, involving the use of an alternating dielectric radio frequency field to devulcanize the elastomers.
2. Description of Related Art
Millions of used tires, hoses, belts, and other vulcanized rubber products are discarded annually after they have been worn-out during their limited service life. These used vulcanized rubber products are typically hauled to a dump because there is very little use for them after they have served their original intended purpose. A limited number of used tires are utilized in building retaining walls, as guards for protecting boats and similar things where resistance to weathering is desirable. Efforts to reclaim scrap vulcanized rubber have primarily included a physical shearing process which is suitable for a rubber which can be mixed with asphalt, forming asphalt rubber. However, a far greater number of tires, hoses, and belts are simply discarded.
During the vulcanization process of rubber, accelerators, promoters, and/or initiators, are used to form large numbers of sulfur crosslinks. After vulcanization, the crosslinked rubber becomes thermoset and cannot be reformed into other products. Thus, vulcanized rubber products generally cannot be simply melted and recycled into new products. The sulfur crosslinks which are present in used vulcanized rubber, such as tire rubber, are deleterious in a subsequent curing process which uses used vulcanized rubber as a component in a new polymer mixture. Formulations of tire rubber which use more than minor amounts of vulcanized rubber result in a brittle cured end product unsuitable for many uses such as automobile or truck tires.
In light of the foregoing, various techniques for devulcanizing rubber have been developed. For example, in one devulcanization process, vulcanized rubber is placed in an organic solvent to recover various polymerized fractions as taught in Butcher, Jr. et al. U.S. Pat. No. 5,438,078. Platz, U.S. Pat. No. 5,264,640 teaches taking scrap rubber from used tires and regenerating the monomeric chemicals which are subsequently recovered. This method uses gaseous ozone to break down the crosslinked structure of the rubber followed by thermal depolymerization in a reaction chamber. Platz et al., U.S. Pat. No. 5,369,215 teaches a similar process in which used tire material may be depolymerized under elevated temperatures and at a reduced pressure to recover the monomeric compounds. Myers et al., U.S. Pat. No. 5,602,186 discloses a process for devulcanizing rubber by desulfurization, comprising the steps of: contacting vulcanized crumb rubber with a solvent and an alkali metal to form a reaction mixture, heating the reaction mixture in the absence of oxygen and with mixing to a temperature sufficient to cause the alkali metal to react with sulfur in the crumb rubber, and maintaining the temperature below that at which thermal cracking of the rubber occurs, thereby devulcanizing the crumb rubber. Hunt et al., U.S. Pat. No. 5,891,926 is directed to a devulcanization process for rubber in which elevated temperatures and pressures are used to partially devulcanize the rubber. Thereafter, a solvent 2-butanol is used to extract the devulcanized rubber from the non-rubber and/or solids component.
Novotny et al., U.S. Pat. No. 4,104,205 discloses a technique for devulcanizing sulfur-vulcanized elastomer containing polar groups which comprises applying a controlled dose of microwave energy of between 915 MHz and 2450 MHz and between 41 and 177 watt-hours per pound in an amount sufficient to sever substantially all carbon-sulfur and sulfur-sulfur bonds and insufficient to sever significant amounts of carbon-carbon bonds. Other patents directed to microwave techniques include Lai et al. U.S. Pat. No. 4,440,488; Hayashi et al., U.S. Pat. No. 4,469,817; Ficker, U.S. Pat. No. 4,665,101; and Wicks et al., U.S. Pat. No. 6,420,457. In general, the application of microwave energy results in uneven heating of the elastomer. As such, the degree to which the elastomeric particles are devulcanized vary within the rubber particle, which is typically most evidenced by different surface and interior properties.
Isayev et al., U.S. Pat. No. 5,284,625 discloses a continuous ultrasonic method for breaking the carbon-sulfur, sulfur-sulfur and, if desired, the carbon-carbon bonds in a vulcanized elastomer. Through the application of certain levels of ultrasonic energy (15 kHz to 50 kHz) in the presence of pressure and optionally heat, it is reported that vulcanized rubber can be broken down. Using this process, the rubber becomes soft, thereby enabling it to be reprocessed and reshaped in a manner similar to that employed with previously uncured elastomers. Other patents directed to ultrasonic devulcanization techniques include Isayev, U.S. Pat. No. 5,258,414 and Roberson et al., U.S. Pat. No. 6,095,440.
Despite the various devulcanization processes known the art, there remains a need to develop improved devulcanization techniques, especially those that are capable of devulcanizing the rubber particles in a relatively uniform manner.